The present invention relates to a spray coating system including a method and apparatus for controlling the spraying of articles by an automatically controlled sprayer as those articles are carried by a conveyor through a spray area.
In such a system, a spray control circuit turns the sprayer on and off at the correct moments and moves it through the spray area in synchronism with the article to be sprayed. In typical prior art systems, the control circuit may also determine the amount of coating material sprayed per unit of time and may control a device that charges the coating material electrostatically. Such control systems advantageously incorporate a freely programmable microprocessor with a memory that will store various programs for different coating procedures. Exemplary of such systems are British Pat. No. 2,013,934 and U.S. Pat. No. 4,357,900.
In order for the system to properly synchronize the operation of the sprayer with the movement of the article to be coated, it must know the position of the article as it moves through the spraying chamber. To this end, the system typically receives timing signals indicative of the speed of movement of the article through the spray area. The timing signals are typically generated by using a pulse generator which generates pulses in response to the motion of the conveyor at a point remote from the spray area. If an initial position of the article is known, and if the length of the conveyor remains constant, these pulses will provide an accurate indication of the instantaneous position of the article to be sprayed as it is moved through the spray area.
The initial position of the article is normally determined using an edge detector which senses the front edge of the article at a predetermined position upstream of the spray chamber. Since the movement of the conveyor will not always be smooth, the article to be painted often swings back and forth. If the article is swinging forward as it approaches the front edge detector, the detector will generate an output signal before the article has reached the desired initial position. If the article is swinging backward at the time it approaches the front edge detector, the front edge detector will generate an output signal after the article reaches the desired position. This can create differences between the actual position of the article as it is moved through the spray area by the conveyor and an apparent position of the article determined by the timing signals.
In addition to these errors, differences in tolerance, especially those created by the longitudinal expansion of the conveyor, will cause alterations in the dimensions of sections of the conveyor. This will also create differences between the actual position of an article as it moved through the spray area by the conveyor and an apparent position of the article determined by the timing signals. Such differences in actual and apparent positions will give rise to errors in the process by which the operation of the sprayer is synchronized with the movement of the article through the spray area. As a result, coating material may be sprayed to one side of the article, areas of the article which should be coated may be missed, etc.
Conveyors employed in conjunction with known spray-coating systems travel at a rate of approximately 6 meters per minute. When using a microprocessor based control circuit to control the operation of the sprayer, approximately 100 ms must be provided for the microprocessor to process a single control step and to prepare to accept a new control step. Therefore, the control circuit can only respond to timing signals (indicative of the speed of the conveyor) having a frequency of no more than about 600 signals per minute.
To allow for some margin of error at 6 meters per minute and to permit operation at a conveyor rate as high as 12 meters per minute, one timing signal should be produced for every two centimeters of conveyor travel. Fewer timing signals per section of conveyor travel would provide too low a resolution to permit accurate coating of the articles because alterations in the coating process could not take place accurately enough with respect to the time taken for the articles to move through the spray area. While it is possible to generate timing signals at high rates in response to the motion of the conveyor or the means for driving the conveyor (i.e. a drive motor) to provide high resolution information concerning the speed at which an article is moving through the spray area, the rate of such signals is too high to be utilized by a microprocessor based control circuit.
As the article is moved by the conveyor through the spray-coating region, the movement of the sprayer must be accurately controlled to be coordinated with the movement of the article to be sprayed. In certain instances, it is desirable for the sprayer to move at the same speed as the article to be coated; it is sometimes desirable for it to move at a greater or lesser speed than the article to be coated and it is sometimes desirable to keep the sprayer stationary while the article moves past it. In all cases, accurate control of the movement of the sprayer is necessary. This has not always been possible with prior art systems.